People who have been exposed to large doses of radiation (such as after the atom bomb) or radiotherapy for cancer treatment have sometimes developed leukaemia, many years later. However, radiation used to treat cancer is not thought to cause leukaemia.

In the past higher doses of radiation were used to treat a number of conditions and some patients did develop second cancers, including leukaemia. Since that time experts have looked very closely at the risks of giving radiotherapy as part of cancer treatment. Perhaps surprisingly there is virtually little evidence that modern day radiotherapy leads to an increased likelihood of developing a further tumour, or leukaemia, when it has been used as part, or all, of treatment for a cancer.

The one exception to this is Hodgkin lymphoma. In this cancer the use of chemotherapy does lead to a small risk of a second cancer developing years later. Although using radiotherapy on its own to treat Hodgkin lymphoma doesn't seem to carry any obvious risk, if radiotherapy is given as well as chemotherapy then there is a greater risk of later cancer development than there would be if chemotherapy only was given.